1. Field of the Invention
The invention relates to a physical vapor deposition system and, more particularly, to an improved target and dark space shield for improving particle performance within such a system.
2. Description of the Background Art
FIG. 1 depicts a simplified, cross-sectional view of a conventional physical vapor deposition (PVD) system 100. The system contains, within a system housing 101, a substrate support assembly 102, a shield ring 104, a plasma shield 106, a dark space shield 108, a collimator 110, and a target 112. These elements of the PVD system are conventionally arranged within the housing 101. When a gas, such as argon, is pumped into region 120 and high voltage is applied between the target 112 and the substrate support 102, the argon forms a plasma within region 120. The plasma sputters the target material which ultimately is deposited upon a substrate 122.
The target 112 is fabricated of a backing plate 114 (typically aluminum) that has a target material to be sputtered (e.g., titanium) diffusion bonded as a target layer 116 upon its surface. Besides the foregoing target, other target designs are used in the art. For example, the target backing plate may be cup or dish shaped, i.e., having a hollow center, rather than a solid plate.
The dark space shield 108, besides supporting the collimator 110, also shields the backing plate from being sputtered. Sputtering the backing plate would generate particles of the backing plate material which would contaminate the substrate. As such, it is important that only the target layer is sputtered and not the backing plate material. As such, the dark space shield is positioned in close proximity (approximately 0.065 inches) to the target. Such a small gap prevents the plasma from leaking into the gap and sputtering the backing plate.
To further illustrate the relationship between the dark space shield and the target, FIG. 1A depicts a detailed, cross-sectional view of the interface area. The target 112 is maintained in a spaced-apart relation relative to the dark space shield by an insulator ring 124. Although the dark space shield and the target are near enough to one another to prevent the plasma from sputtering the backing plate, the gap 126 is large enough to permit sputtered material from the target layer 116 to enter the gap 126 and deposit upon the curved portion 134 of the backing plate 114 and the side wall 128 of the target layer 116. This phenomenon is known in the art as backscatter deposition. The deposition occurs from particles having trajectories illustrated by arrows 130. Assuming a target support surface 136 is horizontally disposed, the deposition occurs in the region where an angle between the target side wall and the target support surface 136 is greater than ninety degrees. As such, deposition in the gap will occur for all particles having trajectories that are on a line-of-sight path with the backing plate and target layer edge. The angle within which particles are line of sight to the gap 126 is shown as reference number 132.
Deposition onto the curved portion 134 of the backing plate 114 and the side wall 128 of the target layer 116 is detrimental to a PVD process. For example, the deposition upon the side wall of the target layer and the curved portion of the backing plate occurs along an oblique angle to these surfaces. As such, when the temperature of the target changes and causes the target to expand or contract, the deposited material (or portion thereof) dislodges from the surface and may fall upon the substrate. Consequently, the substrate would be contaminated.
Therefore, a need exists in the art for an improved target and dark space shield that reduces the deposition of material in the gap between the shield and the target.